The invention is concerned with access equipment by means of which access is gained to elevated locations. More particularly, it is concerned with such access equipment in which a working platform or other load is mounted upon the upper or uppermost of two or more articulated booms and is moved to a desired elevated location by angular movement of the booms about horizontal axes.
A constant search among designers of access equipment of this type is for methods of extending the height of reach and/or the lateral extent of reach (the "outreach") of such equipment while maintaining the overall stability of the structure without, if possible, increasing the area of the equipment base. In particular, the greater the outreach beyond the limits of the base, the greater is the need to provide a counter-balancing force, in terms either of base weight or of the angle at which the lower boom is set.
The greatest outreach using a conventional two-boom design of access equipment is most conveniently achieved by allowing the lower boom to approach a vertical position and extending the upper boom horizontally. However the outreach can then only be reduced in operation by reducing the angle (the degree of verticality) of the lower boom. This causes the lower boom to extend further outwards from the base in the opposite direction and means that the operator must, in adjusting the outreach, also watch closely the movement of the lower boom (the so-called "tail-swing"). In restricted operating areas, the tail-swing may be a considerable constraint on the freedom of movement of the working platform. In public areas such as highways, the projecting of the lower boom may be a serious hazard to traffic.
In order to maintain stability of conventional access equipment during operation, it is important that the equipment be unable to get into an unstable operating position. This is usually achieved either by physically limiting the extent of movement of the booms (for example by restricting the action of the operating rams) or by providing electronic control equipment which prevents dangerous combinations of boom and platform positions from arising.
Against this background, it is an object of the present invention to provide access equipment of the above general type in which height of reach and outreach are maximised while making it possible to avoid altogether the hazard of tail-swing and simultaneously minimising the need for elaborate and costly control equipment.
The access equipment according to the present invention is characterised by two features, namely:
(a) the movements of the upper boom or booms cannot be initiated until the lower boom is locked in an elevated position; and PA1 (b) at least one upper boom is extensible.
The elevating of the booms and if desired the extending thereof may be carried out in conventional manner by the use of hydraulic rams. Thus the interlocking of the various movements to ensure that the upper boom or booms cannot be moved until the lower boom is set may be achieved mechanically and/or hydraulically, although electronic inter-locking is an acceptable alternative if desired. By way of example, the action of the ram which effects elevation of the lower boom may operate a release valve which permits hydraulic fluid to flow to the controls effecting the movements of the other boom or booms.
The lower boom may be required to be elevated to its position of maximum height before the other movements are made available to the operator or, less preferably, elevation to any lesser height and locking of the lower boom in that position may permit such other movements. Thus the lower boom elevating ram may then operate a release valve as aforesaid only when the piston of the ram reaches the maximum limit of its stroke. Alternatively, the boom itself may operate the release valve when the boom reaches its maximum elevation.
In another form of the invention, mechanical interlocking ensures that the upper boom cannot be moved until the lower boom is fixed in its position of maximum elevation.
When the lower boom has been elevated and locked in position, adjustment of the spatial position of the working platform or other load may be achieved solely by adjustment of the elevation and extension of the upper boom or booms. Thus, in the simple case of a two-boom unit, the upper boom may be elevated until it is horizontal and then the outreach is determined exclusively by the extension of the upper boom. Vertical adjustment of the position of the working platform may, in that case, be achieved by varying the angle of elevation of the upper boom.
In a further development of the access equipment according to the present invention, the lower boom may also be extensible, for example telescopic. Extension of the lower boom may be permitted either only as a second step immediately following elevation and locking of that boom or alternatively at any stage after said elevation and locking. By the first arrangement, tail-swing may need to be considered only during initial setting of the lower boom; by the second arrangement, vertical adjustment of the position of the working platform is achievable with minimum effect on the extent of outreach.
Additional extent of movement of the working platform may be achieved by providing more than one upper boom, in which case only one of the upper booms or more than one of them may be extensible.